In recent years, marked advances in miniaturization of mobile communication devices, such as mobile phones and Wireless LAN (Local Area Network) devices, have been achieved, thanks to miniaturization of the various components incorporated therein. One of the components incorporated in a communications device is a band pass filter. A band pass filter suppresses signals having frequencies outside a particular bandwidth, and passes signals having frequencies within the particular bandwidth, with little or no suppression. Band pass filters are widely used. For example, band pass filters may be found in duplexers and diplexers associated with cellular telephones. Band pass filters may also allow an antenna of a cellular phone to transmit at one band of frequencies while receiving at another band of frequencies.
One type of band pass filter is described in U.S. Pat. No. 6,020,799, FIG. 12 of which is reproduced as FIG. 1 herein. Referring to FIG. 1, a band pass filter is comprised of two TEM (Transverse Electromagnetic Mode) resonators. The resonators are connected together in a cascade connection. Each resonator includes a pair of transmission lines including a narrow transmission line of high impedance and a wider transmission line of lower impedance. The narrow transmission line of each resonator may be regarded as a proximal end of the corresponding resonator. The proximal end of each resonator is connected to the proximal end of the other resonator, and is also grounded. The wider transmission line of each resonator, having a lower impedance than either of the narrow transmission lines, may be regarded as a distal end of the corresponding resonator. The distal end of each resonator is “open” in that no direct electric current may flow therethrough. The narrow transmission line of each resonator is coupled electromagnetically to the narrow transmission line of the other resonator, and the wider transmission line of each resonator is coupled electromagnetically to the wider transmission line of the other resonator. Electromagnetic fields generated in each resonator interfere and couple with electromagnetic fields generated in the other resonator, creating what may be described as a “coupling” or an “electromagnetic coupling” between the resonators.
In U.S. Pat. No. 6,020,799, however, the distal end of each TEM mode resonator is capacitively coupled to input and output terminals and to ground. The overall structure provides a substantial amount of capacitive coupling. An inter-stage coupling capacitor is also added to control the coupling as well as the attenuation pole frequency, but throughout the structure, the reactive coupling is entirely capacitive. Since the coupling is entirely capacitive, only limited control is available to place attenuation poles with precision. To mitigate this capacitive reactive coupling, the line distance between the resonators can be reduced, and the attenuation pole frequency of the transmission characteristics can also be adjusted by varying the line distance of the first transmission lines and the line distance of the second transmission lines. But these approaches to mitigating the excessive capacitance of the reactive coupling can only go so far, since photolithographic techniques cannot place the lines more closely together than the photolithographic limit, generally 0.2 mm today. Also, by adjusting the even and odd mode impedance ratio of the transmission line by the distance between lines, the degree of coupling can be changed. Nevertheless, the line distance between the resonators can only be reduced so far.